Interferometry is a technique, which uses patterns (also known as “fringes”) obtained by interference of two beams of light (or any other types of waves) to measure out-of-plane shape/deformation of objects. Based on their relative phase, two waves can interfere constructively or destructively. Constructive interference causes an increase in the intensity of the light (white fringes) and destructive interference causes a decrease in the intensity (dark fringes). An interferogram is an image of these interference patterns and it looks as strips of dark and white regions that can span across the image. Interferometry can also be viewed as a method of encoding that encrypts the three-dimensional information on a two-dimensional image. One commercial application of this is in holograms. The image seen when looking at a properly illuminated hologram changes as the position and orientation of the viewing system changes making the image appear three dimensional. The holographic recording itself is not actually an image though. It is made as structures of either varying intensity, density or profile which have no resemblance to the actual image.
Different techniques have evolved from the basic idea of using interference of waves to measure the out-of-plane height. Based on the frequencies of interfering waves being similar or different these methods can be divided into two main classes, i.e. homodyne or heterodyne. Some examples include Fabry Perot, Mach Zehnder, Fizeau and Michelson Interferometers. The method used as an exemplar of the present invention is the latter one.
The basics of operation of a Michelson interferometer are relatively simple. A collimated beam of light is divided into two identical beams, which combine again after reflecting back from the sample and the reference mirror, creating an interferogram at the CCD camera.
In interferometry and holography, the out-of-plane height information are coded as phase difference between the incident waves. Theoretically, there is no height range limit for this method as long as it is less than the coherence length of the light used and within the focal length of the imaging system but phase data obtained with this method are wrapped into [−π,π] range. This means there is no one-to-one relationship between phase and height which makes decryption (unwrapping) of the phase and getting the height data mathematically challenging and sometimes impossible. It also means that unlike the height information, the phase information stored in the fringes is not continuous but rather in the form of discontinuous strips of information.